1. Field of the Invention
This invention relates to a dielectric filter comprising a plurality of dielectric resonators of which adjacent ones are connected to each other electromagnetically or via a coupling element. More particularly, the present invention relates to a band-pass type dielectric filter having a pole in an attenuation region.
2. Description of the Prior Art
In a band-pass filter, it is sometimes requested by a user than an excellent frequency attenuation should be obtained in a certain region that is separated from the center frequency by a certain degree. To accomplish the aforesaid request, in a dielectric filter comprising a plurality of resonators, whether cavity or dielectric type, of which adjacent ones are connected to each other electromagnetically or via a coupling element, one method is to increase the number of resonator stages.
However, when the number of the resonator stages increases, the filter itself becomes bulky and expensive. Also, as the number of stages increases, the resonant frequency of the TE.sub.11 mde shifts towards the lower frequency region close to the resonant frequency of the wanted mode, TEM mode. Therefore, the TE.sub.11 mode will be rendered as the spurious mode.